Reduction is the process of gaining electrons. In electron transport chain (ETC), electron carriers such as NADH and  donate electrons to the electron carriers in the ETC. These electrons are transported to subsequent molecules. The final acceptor of electrons in ETC is oxygen, which accepts electrons and gets converted into water. Since electrons are being lost from them, NADH and  are oxidized in the ETC. On the other hand, oxygen accepts electrons and is reduced in the electron transport chain.

The question states that oligomycin inhibits ATP synthase. Recall that ATP synthase (found on the inner mitochondrial membrane) generates ATP by transporting protons from the intermembrane space (space between inner and outer mitochondrial membrane) into the mitochondria. Inhibiting this will prevent the transport of protons and will, subsequently, lead to a buildup of protons in the intermembrane space.

Proton pumps are also found on the inner mitochondrial membrane. They function to pump out protons from the inside of mitochondria to the intermembrane space, thereby providing the proton gradient for ATP synthase to generate ATP. Halting ATP synthase will cause proton pump to stop pumping protons into the intermembrane space (due to the increase in protons in intermembrane space).

ATP synthase is the major generator of ATP; therefore, halting ATP synthase via oligomycin will decrease the amount of ATP generated.

The F0 subunit of ATP synthase is where protons flow through to create ATP. This mechanism involves a rotation of the subunit, producing ATP with each turn. ATP synthase is part of oxidative phosphorylation, the greatest ATP producing segment of cellular respiration.

Electrons flow throughout the electron transport chain via redox reactions. They flow from the most negative voltage to the most positive voltage within the chain. Thus the correct flow would be from C (most negative) to B (Less negative) to A (most positive).

Photosystems I and II are each capable of conducting electrons, with photosystem II handing off electrons to photosystem I. This is accomplished by the electron carrier molecule plastocyanin. 

Photosystems I and II are responsible for the light-dependent reactions of photosynthesis. These two photosystems work in tandem to create ATP and NADPH products. ATP is created in photosystem II, while NADPH is created in photosystem I.

Excitation of photosystem II splits water in the thylakoid space into hydrogen and oxygen. The hydrogen then passes through ATP synthase to move down its concentration gradient and into the stroma. Excitation of photosystem I passes electrons to NADP⁺ reductase to convert NADP⁺ to NADPH. Regeneration of NADPH is necessary for the Calvin cycle. 

Electrons excited in photosystem I are accepted by , thus converting  to .  is the reduced form of  and while  acts as an electron acceptor in certain reactions, the light reactions utilize  which has an extra phosphate.  and  are not used to accept electrons in this context. 

In order to keep the Calvin cycle going, the 5-carbon molecule that carbon dioxide attaches to in the first step must be remade at the end of the cycle. This molecule is called ribulose-1,5-bisphosphate, or RuBP.

C3 plants use rubisco to fix carbon dioxide; however, oxygen also competes for binding. C4 plants have evolved to use PEP carboxylase, which only binds carbon dioxide, eliminating competition with oxygen. Furthermore, C4 plants separate carbon fixation and the Calvin cycle by location, but C3 plants do not. The light reactions of C3 and C4 plants are very similar, but C4 plants are more suited for arid climates due to their ability to close their stroma for longer periods of time to prevent water loss.